As a kid I enjoyed tinkering with robots, buggys, etc. The culmination of these was a 6-legged walking robot which ran off an air compressor and a BBC B - an ancient lego-glue-and-breadboard ancestor of this robot (sadly, nothing to do with me), using the definitely cool Shadow Project air muscles).

As an adult I've been unable to resist the similarly cool Lego Mindstorms. Although I haven't had much time to play with this, I can say that it's definitely a high-quality kit. For example, the infrared interface is about 100% more reliable than the one in my laptop. It comes with a point-and-click interface for programming, but you can dump that and use Perl instead (haven't had time to try this yet myself).

Anyway, I had a point (I think). This sort of simple hardware started me off on programming because of the very tangible results, e.g. the project can be to have the robot follow a white line on the floor, or try and trip people up. Depends on the kid of course.

andy.